The present invention relates to a clamping mechanism for a clamping unit of an injection molding machine.
German Pat. No. DE 101 03 983 C1 discloses a clamping mechanism in the form of a screw mechanism including a ball nut for implementing the closing and opening motions of the moving platen, and a locking nut disposed on a common screw shaft. The ball nut and the locking nut are resiliently kept at a distance so that the locking nut engages with play in the threads of the screw shaft, when the platen moves. In other words, the locking nut moves contactless. When the platen is in closing position, the ball nut is shifted in axial direction to such an extent in opposition of a spring force under the action of a clamping force, applied by a clamping force cylinder via the screw shaft, that the threads between the locking nut and the screw shaft come into contact. Subsequently, the locking nut is pressed axially against the moving platen in opposition to a further spring assembly so that the brake gap of a friction brake, disposed between platen and locking nut, is closed. As a result of the thus realized rotationally fixed engagement of the locking nut upon the platen, the screw mechanism is locked and the clamping force bypasses the ball nut and is transmitted via the threaded connection between screw shaft and locking nut to the moving platen.
Although such a clamping mechanism affords an overload protection for the ball nut of a combined drive and locking mechanism, the locking nut can be restrained against rotation only in a very complicated manner. Moreover, the spring assembly is also complicated as it requires the arrangement of several, individual springs with different spring constant to respond successively.
It would therefore be desirable and advantageous to provide an improved clamping mechanism which obviates prior art shortcomings and is simple in construction while still maintaining an effective overload protection, and in particular is characterized by the absence of selectively engageable friction elements or form-fitting elements between locking nut and platen for securing the screw mechanism against reverse rotation.